Make the progression the same as the rangers: namely 1st level spells at 4th, 2nd level spells at 7th, 3rd level spells at 10th and 4th level spells at 13th.
Then as an expansion plan for the existing spellcasting system for all classes take appropriate spells from the cleric (for paladin) and druid (for ranger) 5th and 6th level lists to become the 5th level spells for pal/rng granted at 16th, then repeat with 6th and 7th level spells to become the paladin/ranger 6th level spells, granted at 19th. Also add 7th level spells to bards (taken from the sor/wiz list 8th, 9th) at 19th and 10th level spells to clerics, druids and wizards at 19th and to sorcerors at 20th.